A firefighter, involved in the rescue operation, also inhaled the toxic fumes and was admitted to the hospital, they added.
Yusuf (50) and his two sons -- Jahangir (24)and Izaz (22) -- collapsed while cleaning the sewer, the police said.
As they did not come out of it for a long time, the police and fire department were informed.
One of the firemen, Mahipal, who had entered the sewer, also inhaled the toxic fumes and collapsed, the police said.
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She added that Yusuf and Mahipal were undergoing treatment at the hospital and their condition was critical.
Last week, three men who were manually cleaning a sewer line at Lajpat Nagar had died after inhaling noxious fumes.
The incident was reported three weeks after the death of four men while cleaning a septic tank in south Delhi's Ghitorni.
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